High energy radiation such as that from x-ray undulators and multipole wigglers installed in high energy photon sources such as synchrotrons are increasingly being used in applications of ultra-monochromatic radiation in various fields of science and technology. Monochromatization of the hard x-ray component (5-30 keV) of synchrotron radiation down to the .mu.eV-neV level may be achieved via coherent nuclear resonant scattering. This technique involves a nuclear resonant medium having a coherent response for producing an energy bandpass of .mu.eV-to-neV. However, the nuclear resonant medium also has a non-resonant response (viz. Rayleigh scattering) which, if not suppressed, will generally overwhelm the detection system and lead to a prohibitively poor signal-to-noise ratio. Despite available techniques to suppress non-resonant scattering, it is extremely beneficial to reduce the energy bandpass of the x-ray beam as much as possible before it is incident on the nuclear resonant medium. It is possible to arrange the resonant atoms in a crystal lattice in such a way that for certain reflections only the resonant nuclei scatter in phase. Thus, a perfect sample of such a crystal can suppress a large fraction of the unwanted electron scattering.
It is well known in the prior art that high brightness undulators provide high flux in the resonant bandwidth in the form of a very low divergence beam. Thus, an appreciable portion of the intensity of the incident x-ray beam can be captured before it is made to diverge from a single crystal with a vertical divergence of only .apprxeq.25 microradians. Using dispersive geometry, researchers at Brookhaven National Laboratory have used Si(8 4 0) crystals to achieve 0.09 eV resolution with an angular acceptance of 6 microradians. However, the apparatus employed to achieve this is of considerable size, i.e., 60" high and 24" long. The divergence of x-rays coming from current radiation sources is typically on the order of 100 microradians. The divergence of x-rays from the next generation of synchrotron radiation sources such as the Advanced Photon Source at Argonne National Laboratory will be approximately 25 microradians. Current monochromators are of only limited use in capturing the full intensity of the less diverging x-rays of the next generation of high energy photon sources. A diffractometer for nuclear Bragg scattering is disclosed in "Construction of a Precision Diffractometer for Nuclear Bragg Scattering at the Photon Factory" in Rev. Sci. Instrum., 63(1), January 1992, by Ishikawa et al. The disclosed diffractometer includes a nested pair of crystals in fixed relation with no energy tuning capability. A monochromator system for use in nuclear Bragg scattering is disclosed in "New Apparatus for the Study of Nuclear Bragg Scattering", Nuclear Instruments and Methods in Physics Research, A266 (1988), 329-335, by Siddons et al.
The present invention addresses the aforementioned limitations of the prior art by providing an x-ray monochromator employing, in combination, an asymmetrical channel-cut single crystal of lower order reflection and a symmetrical channel-cut single crystal of higher order reflection in a novel nested geometry which allows for the incident x-ray beam to be collimated by the asymmetrically cut crystal before undergoing high order reflection by the symmetrically cut crystal in an arrangement which affords precise energy tuning.